1. Field of the Invention
The present invention relates to a technology in which a laser beam is converged into a transparent material to change an optical property of the beam-converged portions of the material so as to form a mark within the transparent material.
2. Description of the Related Art
As a method for forming a mark within a transparent material, there have been known several processes disclosed in Japanese National Publication HEI 6-500275, JP-A HEI 7-136782 and JP Patent No. 2810151. Each of these disclosed processes is to converge a laser beam into a transparent material to form a crack within the material by virtue of a non-linear absorption effect. In this way, an opaque portion will result due to the formation of the crack, thereby forming a desired mark within the transparent material. On the other hand, in order to form a clearly visible mark in a transparent material using any one of the aforementioned processes, it is necessary to enlarge the size of a crack. However, once the crack is made large in its size, the strength of the transparent material will become low because the crack can probably extend to the surface of the transparent material, thus bringing about a damage (destruction) to the material.
In view of the above, the inventors of the present invention have tried an improved laser radiation process capable of performing a specific control of a laser beam, so that a crack is formed only in an internal position of a glass substrate even if it has a small thickness, as disclosed in JP-A HEI 11-13896.
Further, the inventors of the present invention have also developed another method which does not need to form a crack, but needs to change the refractive index of an object material so as to form a desired mark, as disclosed in JP-A HEI 11-267861. With the use of this method, since the crack is not formed, it is allowed to further reduce the possibility of causing a damage to an object material, as compared with the aforementioned method disclosed in JP-A HEI 11-13896.
However, in a process for carrying out the above method using a change in the refractive index, a mark will become extremely small in its size. As a result, a specific reading apparatus will become necessary to detect the presence of the small mark. Namely, although the aforementioned method can be suitably used to form a “hidden mark”, it is not suitable for general use in forming a common mark.